Temperature and nutrition as factors in conditioning broodstock of the commercial scallop Pecten fumatus Reeve

Difficulty in obtaining ripe broodstock of the commercial scallop Pecten fumatus from wild populations in New South Wales prompted the development of hatchery conditioning protocols. Microalgal diets, feeding rates and holding temperatures conducive to rapid gonadal growth and development were identified. Results of microalgal clearance rate experiments indicated that the species Pavlova lutheri, Tahitian Isochrysis aff. galbana, Chroomonas salina and Chaetoceros gracilis are ingested by adult scallops at similar rates at 14, 18 and 21°C, but more slowly at 11°C. Maximum (satiation) ingestion rate using mixed diets containing approximately equal numbers of cells of these four species, was estimated as about 6 x 10⁹ cells per day for broodstock in the range 55 to 75 mm shell height. Large numbers of cells of the microalga Tetraselmis suecica passed through the digestive tract of scallops undigested. Egg production rate, gonad size and condition factors were all highest when broodstock were held at 15°C, lowest for scallops maintained at 21°C and of intermediate values at temperatures of 12 and 18°C. Gonad condition and egg production increased as feeding rates were raised from 12.5 to 100% of satiation at all test temperatures in the range 12 to 21°C. Frequent spawning triggered by routine handling of broodstock during these experiments emphasised the need for conditioning equipment and protocols that minimise handling and other disturbance factors. No unplanned spawning occurred at feeding rates of 25% or 12.5% of satiation at 12°C. The use of low temperature and low feeding rate might therefore enable short term stockpiling of broodstock once they attain prime breeding condition. Findings of these experiments were successfully applied to a subsequent large scale conditioning and induced spawning trial.